1. Field of the Invention
The present invention relates to an optical system, in particular an imaging optical system, a sensor for checking value documents with such an optical system, and a method for imaging an object point.
2. Related Art
Value documents are understood in this connection to be sheet-shaped objects that represent for example a monetary value or an authorization and hence should not be producible arbitrarily by unauthorized persons. Hence, they have features that are not easily produced, in particular copied, whose presence is an indication of authenticity, i.e. production by an authorized body. Important examples of such value documents are cards, coupons, vouchers, checks and in particular bank notes. Bank notes for the purposes of the invention are sheet-shaped.
Optical systems in general will hereinafter be understood to be systems that influence optical radiation, in particular changing its ray trajectory. Optical radiation designates here in the known way electromagnetic radiation in the UV and/or IR and/or in particular visible wavelength ranges.
For examining value documents there are frequently employed optical sensors that capture in locally resolved fashion optical properties of a value document to be analyzed and for this purpose image strip-shaped areas on the value document at least partly onto a substantially line-shaped field of receiving elements. The receiving elements form, in dependence on the radiation falling thereon, receive signals which together represent an image of the respective area of the value document. By motion of the value document transversely to the direction of the line or strip, a two-dimensional image of the value document can be captured through sequential capture of such images during the motion past the sensor.
In many applications it is desirable here that the sensor can be of very compact and inexpensive construction. The optical resolution, on the other hand, need not always meet high demands. Sensors with conventional imaging systems which employ only lenses or parabolic mirrors do not always meet these demands to the desired extent upon line-by-line imaging. In the past, self-focusing light guides, so-called SELFOCs, have sometimes been used for this purpose, but their availability is decreasing. This applies in particular to those SELFOCs having back focal distances necessary for achieving necessary depths of sharpness.
The present invention is hence based on the object of providing an optical system for imaging areas of a value document, a corresponding imaging method, and a sensor for capturing optical properties of a value document, which make possible a compact structure without the use of SELFOCs.